The present invention relates to a magnetoresistive sensor employing at least two magnetic thin film layers whose relative orientations of magnetization change in response to a changing externally applied magnetic field according to the giant magnetorestrictive effect. The invention is used particularly for obtaining information stored as magnetic domains on a magnetic storage medium in an information storage and utilization system.
Magnetoresistive (MR) sensors have been known to employ the anisotropic magnetoresistive (AMR) effect in which a component of the resistance in the sensor varies in proportion to the square of the cosine of the angle between the magnetization of the sensor and the direction of sense current flowing through the sensor. More recently, there have been MR sensors employing the giant magnetoresistive (GMR) effect which employ a plurality of layers of magnetic material, wherein a component of the resistance varies in proportion to the cosine to the first power of the angle between the magnetization of two adjacent magnetic layers.
One particular type of GMR sensor has the magnetization of one magnetic layer fixed or pinned by various means, such as: (1) the use of an anti-ferromagnetic material on the opposite side of the one magnetic layer from the other magnetic layer; the exchange coupling between the anti-ferromagnetic material and the ferromagnetic material of the one magnetic layer will fix the magnetization of the one magnetic layer generally independently of the external magnetization that carries the information to be detected; (2) the use of hard and soft layers; and (3) an adjacent permanent magnet layer or an adjacent layer whose magnetization is fixed with an applied current. This type of GMR sensor is generally known as a spin valve. U.S. Pat. No. 5,159,513, issued Oct. 27, 1992 discloses a spin valve wherein the magnetization of one magnetic layer is fixed or pinned by either the use of an adjacent anti-ferromagnetic material or alternatively by a ferromagnetic layer of sufficiently high squareness, high coercivity and high resistance, that is magnetization harder than the other ferromagnetic layer. Furthermore, a double spin valve is known wherein there are three ferromagnetic layers, with the outer two ferromagnetic layers having their magnetization pinned in the same direction, for example by the use of adjacent anti-ferromagnetic materials, leaving the middle ferromagnetic layer with its magnetization free to be affected by the applied external information magnetic fields.
Although the present invention has application broadly to all GMR effect sensors, it is most preferably and most advantageously usable with a spin valve or double spin valve GMR sensor. A particular detailed disclosure of spin valves is set forth in U.S. Pat. No. 5,373,238, issued Dec. 13, 1994.
Furthermore, U.S. Pat. No. 5,301,079 issued Apr. 5, 1994, discloses a GMR effect sensor wherein two ferromagnetic layers each have their magnetizations free, that is not fixed or pinned, and the sensor is so constructed that in response to the applied external magnetic fields carrying the information, magnetization in each layer is oriented at equal and opposite angles with respect to the easy axis, so that magnetization will rotate through substantially equal but opposite angles. Broadly, the present invention can be applied to the construction of one or more of such magnetic layers.
Generally, the directions of magnetization in the ferromagnetic layers may be parallel, that is the same, or they may be anti-parallel, that is extend in alternating directions for adjacent ferromagnetic layers, or at other relative angles. An example of parallel magnetization is set forth in the above mentioned U.S. Pat. No. 5,301,079, and an example of anti-parallel magnetization is disclosed in U.S. Pat. No. 5,313,186, which employs the principle that the direction of magnetization is dependent upon the thickness of non-magnetic layers between the ferromagnetic layers. Further, the direction of magnetization in adjacent ferromagnetic layers may be at various angles to each other as set forth in U.S. Pat. No. 5,287,238, for example. A detailed disclosure of parallel and anti-parallel magnetization for the GMR effect is set forth in White, Robert L., "Giant Magnetoresistance: A Primer", IEEE Transactions on Magnetics, Vol. 28, No. 5, September 1992, pages 2482-2487.
A thin film magnetic or ferromagnetic layer as used in a sensor of the AMR or GMR effect type is defined herein as a homogeneous single layer film or a multilayer film wherein the individual layers of the multilayers may be the same or different material, but in either event the thin film magnetic or ferromagnetic layer functions as a single entity in the sensor to provide a single magnetization direction that angularly moves relative to the sense current (AMR) or relative to the magnetization of another thin film magnetic or ferromagnetic layer so that the cosine to the second power (AMR) or the cosine to the first power (GMR) of such angle is an indication of the strength of an externally applied information containing variable magnetic field.
In order to function, these two magnetic layers are sufficiently separated, preferably by a metallic non-magnetic layer. The above-mentioned article "Giant Magnetoresistance: A Primer" states that to obtain a GMR effect, it is necessary that the thickness of the thin film layers must be less, preferably a fraction, of the mean free path of an electron in the array of thin film ferromagnetic layers separated by the thin film non-magnetic layer.
A multilayer thin film ferromagnetic layer is disclosed in Ultrathin Magnetic Structures II, "Measurement Techniques and Novel Magnetic Properties", Springer-Verlag, pages 174-194. More particularly, this document discloses a spin valve having an anti-ferromagnetic layer (FeMn), a first magnetic layer that is a multilayer of NiFe film and a Co film, a separation non-magnetic thin film layer of Cu, and a second thin film magnetic layer as a multilayer of a Co film and a NiFe film. It is disclosed that in the multilayer, the relative positions of the Co and the NiFe thin films may be reversed. The present invention relates to one or more of the thin film magnetic layers being a multilayer.
The above-cited documents disclose many different materials to be used as substrates, coupling layer, bonding layer, thin film magnetic layers, non-magnetic separation layer, anti-ferromagnetic layer, oxidation protection and passivation layers, magnetic shielding, and conductive layers. Further, these documents suggest different thicknesses and discuss other properties of the materials.
In the AMR and the GMR type sensors, a sensing electrical current is passed through the layers, and a voltage across the layers (direct) or across a resistance in parallel or series with the layers (indirect) is then preferably sensed as an indication of the information. Optionally, a non-information magnetic field may be passed longitudinally through the layers to reduce or eliminate Barkhausen noise. While the above mentioned distinctions between the AMR effect and the GMR effect sensors use the cosine of the angle as an example, it is understood that such is not restrictive, as it has been recognized that, of course, the sine of the angle may be used.
In general, soft magnetic properties are desirable for the magnetic layer whose magnetization is free to rotate in response to the applied magnetic field. As used herein, soft magnetic properties mean low coercivity, low anisotropy field and low magnetostriction. A low coercivity may be for example 30 Oe or less. Generally a material such as Co has a high coercivity, for example about 30 Oe higher than the about 30 Oe coercivity of NiFe that is commonly used as the ferromagnetic layer material. These low and high terms are well defined in the art, particularly as used in the above-cited documents, and its disclosure with respect thereto is incorporated herein.
A large magnetoresistance change ratio (MR ratio) is preferable for magnetoresistive heads in terms of achieving a large dynamic range. It is a feature of the GMR effect sensor that the MR ratio is many times that of the AMR effect sensor.